This invention generally relates to a surface light source and, in particular, to a surface light source which is capable of transforming light from a point light source to a substantially surface light source.
The structure of a prior art surface light source is shown in FIGS. 10 through 12. Such a surface light source has been used for a front or rear light source 1 of a liquid crystal display device. The light source 1 includes a pair of light-emitting diodes 2 that emit point-like light, or substantially point light (collectively called xe2x80x9cpoint lightxe2x80x9d) L1. The light-emitting diodes 2 are connected to a flexible printed circuit board 3 through which electric energy is supplied to the diodes 2.
Since the diodes 2 emit point light L1, a light guide pipe 4 is provided to transform such point light L1 into line-like light or substantially linear light (collectively called xe2x80x9clinear lightxe2x80x9d) L2. The light guide pipe 4 is made of a resin molded column member and is rectangular in cross section. The diodes 2 are provided at both ends of the longitudinal axis of the light guide pipe 4 and face each other as shown in FIG. 10.
The light guide pipe 4 has a plurality of prisms 5 on one side along its longitudinal direction to transform point light L1 into linear light L2. A light guide plate 6 is provided on the other side opposite to the prisms 5 of the light guide pipe 4. The light guide plate 6 is optically coupled to the light guide pipe 4 at the sides where the plate 6 and pipe 4 face each other. The light guide plate 6 further transforms the linear light L2 from the light guide pipe 4 into surface-like light or substantially surface light (collectively call xe2x80x9csurface lightxe2x80x9d) L3. Thus, as shown in FIG. 12, the upper (front) surface of the light guide plate 6 is provided with special prisms 7 while the back surface has a reflection film not shown but made by coating or by applying an evaporation (vapor deposition) process.
Each longitudinal side surface of the light guide pipe 4 except that with the prisms 5 is covered by a high reflective metal plate reflector 8 as shown in FIGS. 10 and 11. This reflector 8 reflects incident light, with high reflection, to the inside of the light guide pipe 4. Such incident light is derived from reflection and diffusion of the light L1 from the diodes 2 to other directions than that to the light guide plate 6.
The incident light L1 from the diodes 2 to the light guide pipe 4 is transformed into linear light L2 directed to the light guide plate 6 by the prisms 5 of the light guide pipe 4 as shown in FIG. 11 because the prisms 5 are wedge-like in shape and are formed on the side of the light guide pipe 4 opposite to the one facing the light guide plate 6.
As shown in FIG. 12 by dotted lines and an arrow, the linear light L2 propagating from the light guide pipe 4 to the inside of the light guide plate 6 is incident on the prisms 7 formed on the upper surface of the light guide plate 6 and is transformed into surface light L3. The surface light L3 is projected onto a reflection or transparent type liquid crystal display panel not shown but provided underneath the light guide plate 6 by the prisms 7.
The light L3 projected onto the reflection type liquid crystal display panel, on one hand, is reflected back by a reflecting electrode not shown but provided therein, passes through the inside of the light guide plate 6 and comes to the eyes of an operator. The light L3 projected onto the transparent type liquid crystal display panel, on the other hand, is made use of as transparent light from the surface opposite to the one provided with the prisms 7. The point light L1 from the diodes 2 is transformed into linear light L2. Such linear light L2 is eventually transformed into substantially surface light L3 so that it is visible to, and is recognized by, an operator as entirely uniform brightness light for a liquid crystal display panel.
In summary, where the diodes 2 are used as point light sources, the light L1 emitted from the diodes 2 is transformed into linear light L2 by the light guide pipe 4 and the linear light L2 is further transformed into surface light L3 by the light guide plate 6. Since a liquid crystal display panel incorporated into the surface light source absorbs or scatters the surface light L3, an operator recognizes it.
The surface light L3 visible to the eyes of an operator is subject to a series of the light transforming processes so that it loses mostly its electric energy supplied from the flexible printed circuit board 3 to the diodes 2. Thus, the prior art surface light source is low in brightness so that it is not satisfactory to customers.
Further, since the diodes 2 are not concentrically disposed with respect to the display area, it is known that the display area in the center of the liquid crystal display panel becomes lower in brightness as a place of the display area is farther in distance from the diodes 2. Such a difference in brightness depending on a place of the display area is recognized by an operator as uneven brightness and the surface light source significantly loses its product value.
There have been various methods to improve the uneven brightness. The configurations of the prisms 7 formed on the upper surface of the light guide plate 6 are varied gradually as the prisms 7 become farther in distance from the light guide pipe 4 thereby to make a quantity of light uniformly supplied to the entire surface of the refection type liquid crystal display panel, i.e., a uniform brightness distribution of the display area.
In short, the point light L1 from the diodes 2 is greater in quantity at a closer place to the diodes 2 but is less at a farther place from them. This causes uneven brightness distribution of the light on the display panel. Thus, the prisms 7 of the light guide plate 6 have specific configurations, respectively, in accordance with the distance from the light guide pipe 4 as set forth above. Alternatively, incident light quantity from the light guide pipe 4 to each of the prisms 7 of the light guide plate 6 is adjusted to reduce difference in brightness of the display area.
However, those methods have not always brought about uniform brightness distribution on the display panel. In addition, the light guide plate 6 cannot increase its brightness unless the light guide pipe 4 is improved to supply a sufficient quantity of light to the light guide plate 6.
The present invention is to provide a surface light source with substantially improved uniform brightness.
Briefly, a surface light source of the invention includes a light source to emit point light, a first light transforming member to transform the point light into linear light and a second light transforming member to transform the linear light into surface light. The first light transforming member includes a concave portion or lens to receive the point light from the light source and to distribute the same to the first light transforming member. Since the point light is distributed by the concave lens of the first light transforming member, it is efficiently transformed to the surface light by the second light transforming member so that the surface light is improved to be substantially uniform in brightness.